1. Field of the Invention
The present invention relates to a pipe joint for a refrigeration cycle, used for joining refrigerant passages of a refrigeration cycle.
2. Description of the Related Art
Recently, a refrigeration cycle using carbon dioxide (CO2), as a refrigerant, has been developed instead of using chlorofluorocarbons. In the system using the CO2 refrigerant, pipes are used for joining refrigerator devices, and pipe joints are used for joining pipes and refrigerator devices or joining pipes, in a manner similar to a conventional system. The applicant has filed a pipe joint used in a conventional refrigeration cycle apparatus previously.
FIG. 11 is a sectional view of a conventional refrigeration cycle pipe joint 7. As shown in the drawing, the pipe joint 7 is composed of a male joint portion 8 and a female joint portion 9, and has a cylindrical sealing structure in which an O-ring 11 is basically used. A cylindrical fitting projection 81 is formed in the male joint portion 8, and the fitting projection 81 is provided with a rectangular groove 81b. The O-ring 11 is fitted in the rectangular groove 81b. 
Because the working pressure of the CO2 refrigerant is high, in order to prevent the o-ring 11 from riding on the fitting projection, a backup ring 10 is provided in the rectangular groove 81b on the side opposite to the pressure receiving surface of the O-ring 11. FIG. 12 is a perspective view of a conventional backup ring 10. The backup-ring 10 is generally made of, resin, or the like, having a ductility less than that of the O-ring 11.
The backup ring 10 cannot be extended, in contrast to the O-ring 11, and, accordingly, is provided with a cut called a bias cut 10a, to be fitted in the rectangular groove 81b beyond a thick portion 81c. FIG. 13A is an explanatory side view of an operation of attaching the backup ring 10 and the O-ring 11 to a conventional male joint portion 8. FIG. 13B is a side view of an assembled male joint portion 8.
A cylindrical fitting recess 91, into which the fitting projection 81, the backup ring 10 and the O-ring 11 are inserted, is formed in the female joint portion 9. The fitting projection 81 is fitted in the fitting recess 91, to join the refrigerant passages, and to prevent the refrigerant from leaking to the outside (to the atmosphere). FIG. 13A is a sectional view of a conventional refrigeration cycle pipe joint 7 in a joined state. FIG. 13B is an enlarged sectional view taken along the line F-F in FIG. 13A. Reference numerals in the drawings, which are not described above, correspond to reference numerals which will be described later in embodiments of the present invention.
However, when the above-described structure of the pipe joint is used in the CO2 refrigerant system, the CO2 refrigerant cannot be sufficiently prevented from leaking, by only the O-ring 11, because a permeability coefficient of the CO2 refrigerant is large and, accordingly, the CO2 refrigerant permeates through a rubber of the o-ring. In addition, the backup ring has the bias cut and, accordingly, an excellent refrigerant sealing property cannot be expected (see FIG. 13B).